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<meta property="og:description" content="前言函数，相信这个名词大家都不会陌生了。 它承载着一个编程语言的精髓。 可以说，前面的基础语法决定了你是否能用javascript，而函数，决定了你是否会用javascript，它甚至会影响你的编程水平。 我们为什么需要函数？ 提高代码的复用性和拓展性，编写代码就应该遵循高内聚、低耦合的原则。 123456789const a &#x3D; 1;if (a &gt; 0)&amp;#123;    console.">
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            <h1 style="display: none">JavaScript函数</h1>
            
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                    本文最后更新于：2023年5月23日 晚上
                  
                
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                <h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>函数，相信这个名词大家都不会陌生了。</p>
<p>它承载着一个编程语言的精髓。</p>
<p>可以说，前面的基础语法决定了你是否能用javascript，而函数，决定了你是否会用javascript，它甚至会影响你的编程水平。</p>
<p>我们为什么需要函数？</p>
<p>提高代码的复用性和拓展性，编写代码就应该遵循<em><strong>高内聚、低耦合</strong></em>的原则。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">const</span> a = <span class="hljs-number">1</span>;<br><br><span class="hljs-keyword">if</span> (a &gt; <span class="hljs-number">0</span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&quot;我要执行了&quot;</span>);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&quot;执行结束了&quot;</span>);<br>&#125;<span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (a == <span class="hljs-number">1</span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&quot;我要执行了&quot;</span>);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&quot;执行结束了&quot;</span>);<br>&#125;<br></code></pre></td></tr></table></figure>

<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">test</span>(<span class="hljs-params"></span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&quot;我要执行了&quot;</span>);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&quot;执行结束了&quot;</span>);<br>&#125;<br><br><span class="hljs-keyword">const</span> a = <span class="hljs-number">1</span>;<br><br><span class="hljs-keyword">if</span> (a &gt; <span class="hljs-number">0</span>)&#123;<br>    <span class="hljs-title function_">test</span>()<br>&#125;<span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (a == <span class="hljs-number">1</span>)&#123;<br>    <span class="hljs-title function_">test</span>()<br>&#125;<br></code></pre></td></tr></table></figure>


<h1 id="函数声明"><a href="#函数声明" class="headerlink" title="函数声明"></a>函数声明</h1><h2 id="直接声明"><a href="#直接声明" class="headerlink" title="直接声明"></a>直接声明</h2><p>函数声明</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">test</span>(<span class="hljs-params"></span>)&#123;<br><br>&#125;<br></code></pre></td></tr></table></figure>

<h2 id="匿名函数"><a href="#匿名函数" class="headerlink" title="匿名函数"></a>匿名函数</h2><p>也叫<strong>函数表达式</strong></p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">const</span> test = <span class="hljs-keyword">function</span>(<span class="hljs-params"></span>)&#123;<br><br>&#125;<br></code></pre></td></tr></table></figure>

<h2 id="箭头函数"><a href="#箭头函数" class="headerlink" title="箭头函数"></a>箭头函数</h2><p>es6新语法，推荐使用，不存在 <strong>函数提升</strong> 问题</p>
<p>函数提升涉及到<strong>预编译</strong>的知识。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">const</span> <span class="hljs-title function_">test</span> = (<span class="hljs-params"></span>)=&gt;&#123;<br><br>&#125;<br></code></pre></td></tr></table></figure>

<h1 id="函数返回值"><a href="#函数返回值" class="headerlink" title="函数返回值"></a>函数返回值</h1><p>return 是函数内部想要往外部传递数据的关键字，这个关键字可以传递一个数据到外界</p>
<p>在return执行了以后，函数就结束了，所以不要在return后面写任何代码，没有作用。</p>
<p>如果没有手动定义return，javascript会默认在函数的代码块最后加上<code>return ;</code></p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">func1</span>(<span class="hljs-params">num1,num2</span>)&#123;<br>    <span class="hljs-keyword">return</span> num1 + num2;<br>&#125;<br><br><span class="hljs-keyword">var</span> ret = <span class="hljs-title function_">func1</span>(<span class="hljs-number">100</span>, <span class="hljs-number">50</span>);<br><span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(ret);<br></code></pre></td></tr></table></figure>

<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-comment">// 箭头函数的简写返回值</span><br><span class="hljs-comment">// 如果箭头函数只有一行，那么不需要写花括号和return即可返回</span><br><span class="hljs-comment">// 类似python中的lambda</span><br><br><span class="hljs-keyword">const</span> <span class="hljs-title function_">func1</span> = (<span class="hljs-params">num1, num2</span>)=&gt;num1 + num2<br><br><span class="hljs-keyword">var</span> ret = <span class="hljs-title function_">func1</span>(<span class="hljs-number">100</span>, <span class="hljs-number">50</span>);<br><span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(ret);<br></code></pre></td></tr></table></figure>

<h1 id="函数参数"><a href="#函数参数" class="headerlink" title="函数参数"></a>函数参数</h1><h2 id="位置参数"><a href="#位置参数" class="headerlink" title="位置参数"></a>位置参数</h2><p>就是按照位置，一个一个传递的参数。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">func1</span>(<span class="hljs-params">num1,num2</span>)&#123;<br>    <span class="hljs-keyword">return</span> num1+num2;<br>&#125;<br><br><span class="hljs-keyword">var</span> ret = <span class="hljs-title function_">func1</span>(<span class="hljs-number">100</span>,<span class="hljs-number">50</span>);  <span class="hljs-comment">// 100 传给num1  50传给num2</span><br><span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(ret);<br></code></pre></td></tr></table></figure>

<p>注意，在JavaScript中，<strong>实参</strong>的个数不一定与<strong>形参</strong>的个数一致。</p>
<ul>
<li>实参，实际传递的参数</li>
<li>形参，定义的参数</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">test</span>(<span class="hljs-params">a, b, c</span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(b);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(c);<br>&#125;<br><br><span class="hljs-title function_">test</span>(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>)<br><br><span class="hljs-comment">// a: 1</span><br><span class="hljs-comment">// b: 2</span><br><span class="hljs-comment">// c: undefined</span><br><br><span class="hljs-title function_">test</span>(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">4</span>)<br><br><span class="hljs-comment">// a: 1</span><br><span class="hljs-comment">// b: 2</span><br><span class="hljs-comment">// c: 3</span><br><span class="hljs-comment">// 多出的参数会被忽略</span><br></code></pre></td></tr></table></figure>

<h2 id="默认参数"><a href="#默认参数" class="headerlink" title="默认参数"></a>默认参数</h2><p>就是有一个或多个参数，可以设定默认值，当有参数传递时，再取对应的实参值。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">test</span>(<span class="hljs-params">a=<span class="hljs-number">0</span>, b=<span class="hljs-number">1</span>, c=<span class="hljs-number">3</span></span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(b);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(c);<br>    <br>&#125;<br><br><span class="hljs-title function_">test</span>()<br><br><span class="hljs-comment">// a: 0</span><br><span class="hljs-comment">// b: 1</span><br><span class="hljs-comment">// c: 3</span><br><br><span class="hljs-title function_">test</span>(<span class="hljs-number">4</span>, <span class="hljs-number">5</span>)<br><br><span class="hljs-comment">// a: 4</span><br><span class="hljs-comment">// b: 5</span><br><span class="hljs-comment">// c: 3</span><br></code></pre></td></tr></table></figure>

<h2 id="arguments"><a href="#arguments" class="headerlink" title="arguments"></a>arguments</h2><p>实参列表</p>
<p>JavaScript还有一个免费赠送的关键字arguments，它只在函数内部起作用，并且永远指向当前函数的调用者传入的所有参数。arguments类似Array, 但它不是一个Array</p>
<p>注意，它不能在<strong>箭头函数</strong>中使用</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params">x</span>) &#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&#x27;x = &#x27;</span> + x); <span class="hljs-comment">// 10</span><br>    <span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i=<span class="hljs-number">0</span>; i&lt;<span class="hljs-variable language_">arguments</span>.<span class="hljs-property">length</span>; i++) &#123;<br>        <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">&#x27;arg &#x27;</span> + i + <span class="hljs-string">&#x27; = &#x27;</span> + <span class="hljs-variable language_">arguments</span>[i]); <span class="hljs-comment">// 10, 20, 30</span><br>    &#125;<br>&#125;<br><span class="hljs-title function_">foo</span>(<span class="hljs-number">10</span>, <span class="hljs-number">20</span>, <span class="hljs-number">30</span>);<br></code></pre></td></tr></table></figure>
<p>利用arguments，你可以获得调用者传入的所有参数。也就是说，即使函数不定义任何参数，还是可以拿到参数的值：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params"></span>) &#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable language_">arguments</span>);<br>&#125;<br><span class="hljs-title function_">foo</span>(<span class="hljs-number">10</span>, <span class="hljs-number">20</span>, <span class="hljs-number">30</span>);<br></code></pre></td></tr></table></figure>

<p>实参列表与实参本身的映射关系: 我变你也变</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params">a</span>) &#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a);  <span class="hljs-comment">// 10</span><br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable language_">arguments</span>[<span class="hljs-number">0</span>]);  <span class="hljs-comment">// 10</span><br>    <br>    <span class="hljs-variable language_">arguments</span>[<span class="hljs-number">0</span>] = <span class="hljs-number">20</span><br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a) <span class="hljs-comment">// 20</span><br><br>    a = <span class="hljs-number">30</span><br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable language_">arguments</span>[<span class="hljs-number">0</span>]) <span class="hljs-comment">// 30</span><br>&#125;<br><span class="hljs-title function_">foo</span>(<span class="hljs-number">10</span>);<br></code></pre></td></tr></table></figure>

<p>建议映射关系的前置条件：实参数需要对应</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">function</span> <span class="hljs-title function_">foo</span>(<span class="hljs-params">a, b</span>) &#123;<br>    <span class="hljs-comment">// 一个实参，所以实参列表arguments的长度为1</span><br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable language_">arguments</span>[<span class="hljs-number">1</span>]); <span class="hljs-comment">// undefined</span><br><br>    <span class="hljs-comment">// 强行修改参数b也无效，因为实际的实参列表长度就只是1.</span><br>    <span class="hljs-comment">// 参数b因为没有被传递，所以不能建议映射关系</span><br>    b = <span class="hljs-number">2</span><br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable language_">arguments</span>[<span class="hljs-number">1</span>]); <span class="hljs-comment">// undefined</span><br>&#125;<br><span class="hljs-title function_">foo</span>(<span class="hljs-number">10</span>);  <span class="hljs-comment">// 这个时候只传递一个实参</span><br></code></pre></td></tr></table></figure>

<h1 id="闭包"><a href="#闭包" class="headerlink" title="闭包"></a>闭包</h1><p>闭包实际上就是函数内再定义一个函数，用来修改外函数的私有变量，从而保证变量安全。</p>
<p>要了解闭包，首先要知道：</p>
<ul>
<li>作用域</li>
<li>预编译</li>
<li>作用域链</li>
</ul>
<p>这三类知识，我们后续再详细介绍。</p>
<h1 id="立即执行函数"><a href="#立即执行函数" class="headerlink" title="立即执行函数"></a>立即执行函数</h1><p>执行一次，马上销毁，javascript特有语法。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><code class="hljs javascript">(<span class="hljs-keyword">function</span> (<span class="hljs-params">a, b</span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(b);<br>&#125;(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>))<br><br>(<span class="hljs-keyword">function</span> <span class="hljs-title function_">test</span>(<span class="hljs-params">a, b</span>)&#123;<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(a);<br>    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(b);<br>&#125;(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>))<br><br><span class="hljs-title function_">test</span>() <span class="hljs-comment">// error, 因为函数已经被销毁了，即使声明了函数变量，但调用时还是会报错。</span><br><br></code></pre></td></tr></table></figure>

<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><code class="hljs javascript"><span class="hljs-keyword">var</span> sum =  (<span class="hljs-keyword">function</span> (<span class="hljs-params">a, b, c</span>)&#123;<br>    <span class="hljs-keyword">return</span> a + b + c<br>&#125;(<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>))<br><br><span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(sum);<br></code></pre></td></tr></table></figure>
                
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